Spectral sensitivity of the photoreceptors responsible for phase shifting the circadian rhythm of activity in the bat, Hipposideros speoris

Abstract

1. The spectral sensitivity of the photoreceptors responsible for phase shifting the circadian rhythm of flight activity in the bat,Hipposideros speoris was investigated. For this purpose we studied the phase shifts evoked with 15 min and 2.77 h pulses of monochromatic light at various phases of the rhythm freerunning in DD. 2. A PRC for the circadian rhythm of flight activity inH. speoris was constructed with white light pulses (1,000 lx for 15 min) against DD background (Fig. 1). In the first set of experiments 15 min monochromatic light pulses of varying intensities were administered to two phases of the rhythm: the phase of the rhythm at which maximal phase advances occur CT 4, and the phase of the rhythm at which maximal phase delays occur CT 18. The intensities of the 15 min monochromatic light pulses required to produce 50% of the phase shifts evoked with white light pulses (1,000 lx for 15 min) at these two phases were determined. The spectral sensitivity curve for advance phase shifts has a maximum at the wavelength 520 nm and the spectral sensitivity curve for delay phase shifts has a maximum at the wavelength 430 nm (Fig. 5). 3. In the second set of experiments 2.77 h monochromatic light pulses of equal energy of 100 μW/cm² were used. We studied the wavelength dependent phase shifts at four phases of the rhythm: CT 2, CT 4, CT 12 and CT 18. The pulses of 430 and 520 nm evoked unequivocal delay and advance phase shifts, respectively, at all four phases (Fig. 7). These results suggest that at this photopic level of pulse energy, there might be a clear antagonism between the two photoreceptor classes, one having a maximum at the wavelength 430 nm and the other having a maximum at the wavelength 520 nm. 4. We suggest that there may exist two different classes of photoreceptors in the retinas ofH. speoris. The S photoreceptors (short wavelength sensitive) having a maximum at the wavelength 430 nm and the M photoreceptors (middle wavelength sensitive) having a maximum at the wavelength 520 nm that mediate delay and advance phase shifts, respectively.